Conventionally, a liquid raw material vaporization feeder for feeding a raw material fluid to a semiconductor manufacturing device for metal organic chemical vapor deposition (MOCVD), for example, has been proposed (e.g., Patent Documents 1 to 3).
In a liquid raw material vaporization feeder of this type, a liquid raw material, such as TEOS (tetraethyl orthosilicate), is heated and vaporized in a vaporization chamber, and the vaporized gas is fed to a semiconductor manufacturing device at a specific flow rate controlled by a flow controller. Then, in order to compensate for the loss of the liquid raw material due to vaporization, it is necessary to detect the liquid level of the liquid raw material and perform feeding to compensate for the decrease, thereby controlling the liquid level.
As a method of detecting the liquid level of a liquid raw material, for example, a pressure-sensitive liquid level detector that monitors a pressure decrease in a vaporizer, thereby detecting a loss of the liquid raw material in the vaporizer due to vaporization (Patent Document 2, etc.) and a thermal liquid level detector that utilizes the difference in heat dissipation constant between the liquid phase and the gas phase (Patent Documents 4 to 6, etc.) are known.
In a thermal liquid level detecting device of this type, as shown in FIG. 15, two protection tubes 3, having enclosed therein resistance temperature detectors R1 and R2 made of platinum or the like, respectively, are vertically inserted into a container 21, and a relatively large constant current I1 (heating current) is passed through one resistance temperature detector R1 so that the resistance temperature detector R1 is maintained at a temperature higher than the ambient temperature by self-heating, while a small constant current I2 (current for ambient temperature measurement) that allows for ambient temperature measurement and causes negligible heating is applied to the other resistance temperature detector R2.
As a result, the resistance temperature detector R1 through which the large current I1 is passed generates heat. At this time, because the heat dissipation constant in the case where the resistance temperature detector is in the liquid phase L is larger than the heat dissipation constant in the case where it is in the gas phase V, the temperature of the resistance temperature detector in the gas phase V is higher than in the case where it is in the liquid phase.
Then, this means that a resistance temperature detector in the gas phase has a higher resistance than a resistance temperature detector in the liquid phase. Accordingly, by observing the difference between the voltage output of the resistance temperature detector R1 through which a large current is passed and the voltage output of the resistance temperature detector R2 through which a minute current is passed, it can be judged whether the resistance temperature detectors are above or below the liquid level. That is, in the case where the difference is small, the resistance temperature detectors can be judged as being below the liquid level, while in the case where the difference is large, the resistance temperature detectors can be judged as being above the liquid level.
FIG. 16 shows an example of a liquid level detection circuit. A constant current is applied to resistance temperature detectors R1 and R2 from a power supply Vcc through constant current circuits S1 and S2. The detection circuit is configured such that a current larger than in the constant current circuit S2 passes through the constant current circuit S1. That is, a small current that allows for ambient temperature measurement and causes negligible heating passes through the resistance temperature detector R2, while a relatively large current having a larger current value than in the resistance temperature detector R2 passes through the resistance temperature detector R1 so that the resistance temperature detector R1 is heated to a high temperature. The terminal voltage V1 of the resistance temperature detector R1 and the terminal voltage V2 of the resistance temperature detector R2 are input to the inverting input and noninverting input of a differential amplifier circuit D, respectively, and, from the differential amplifier circuit D, a voltage signal corresponding to the voltage difference between the terminal voltages V1 and V2 (V1−V2) is input to a comparator C. The comparator C compares the voltage difference with the reference voltage V3 determined by voltage dividing resistors R3 and R4.
When the resistance temperature detector R1 is present in the liquid phase, the temperature rise of the resistance temperature detector R1 relative to the ambient temperature is smaller than the temperature rise in the gas phase. As a result, the output voltage of the differential amplifier circuit D, which is equivalent to the difference from the voltage signal of a magnitude corresponding to the ambient temperature emitted from the resistance temperature detector R2 that is also in the liquid phase, is smaller than the reference voltage, resulting in a low-level output from the comparator C. Meanwhile, when the liquid level decreases and the resistance temperature detector R1 is exposed to the gas phase, the temperature rise relative to the ambient temperature is the temperature rise in the gas phase. Therefore, the output voltage of the differential amplifier circuit D, which is equivalent to the difference from the voltage signal of a magnitude corresponding to the ambient temperature emitted from the resistance temperature detector R2 that is also in the gas phase, is larger than the reference voltage, resulting in a high-level output from the comparator C. When the output of the comparator C is high level, it is judged that the resistance temperature detectors R1 and R2 are in the gas phase, while when the output of the comparator C is low level, it is judged that the resistance temperature detectors R1 and R2 are in the liquid phase.
When the terminal voltages V1 and V2 are determined, according to the Ohm's law, the resistances of the resistance temperature detectors R1 and R2 can be calculated from the current values I1 and I2. When the resistances of the resistance temperature detectors R1 and R2 are obtained, and the rates of change of resistance with respect to temperature of the resistance temperature detectors R1 and R2 are known, the temperatures of the resistance temperature detectors R1 and R2 can be derived. Therefore, in a liquid level detection circuit, instead of comparing the voltage outputs of the resistance temperature detectors R1 and R2, it is also possible to compare the resistances of the resistance temperature detectors R1 and R2 to perform judgement. Alternatively, utilizing the rates of change of resistance with respect to temperature of the resistance temperature detectors R1 and R2, it is also possible that the temperatures of the resistance temperature detectors R1 and R2 are measured from the respective resistances, and the temperatures are compared to perform judgement. In the case of platinum, the resistance is 100Ω at 0° C., and it increases